JP4028332B2 - Light intensity modulator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light intensity modulation device in an optical transmission device used in an optical communication system such as a WDM system.
[0002]
[Prior art]
There is a light intensity modulator as an external modulator used in an optical communication apparatus. As this light intensity modulator, for example, there is a Mach-Zehnder type LN modulator using LiNbO ₃ . FIG. 13 is a block diagram showing a schematic configuration of an optical transmission apparatus using an LN modulator. In FIG. 13, an LN modulator 3 is connected to a laser diode (LD) 1 through an optical fiber 2, and an optical signal input through a connection point P1 is input through a connection point P2. Intensity modulation is performed by an electrical signal S2 of 10 Gbps. The intensity-modulated optical signal is output through the optical fiber 4 connected to the connection point P3. Here, the LN modulator 3 is provided with a monitor photodiode (PD) 3a, and an LN control circuit CC for controlling the bias voltage based on the signal S105 monitored by the monitor PD 3a.
[0003]
As shown in FIG. 14, the LN modulator 3 draws a modulation curve that is a function of cos as the voltage dependence of the optical output. The electric signal S2 is applied as a drive voltage Vπ between the maximum value and the minimum value of the modulation curve L. In this case, a bias voltage Vb is applied to the electric signal S2 so that the midpoint between the maximum value and the minimum value of the modulation curve L is the drive point Pop. The bias voltage Vb is preferably small.
[0004]
However, as a characteristic of the LN modulator 3, the bias voltage Vb has a thermal drift due to temperature and a DC drift with time, and as a result, the operating point Pop changes from the middle point of the modulation curve L to the light output direction (vertical direction). It shifts and an efficient modulation operation cannot be performed. For this reason, feedback control of the bias voltage Vb is performed using the above-described LN control circuit CC so that the operating point Pop is positioned at the midpoint between the maximum value and the minimum value of the modulation curve L. For example, when the modulation curve L moves to a higher voltage, the LN control circuit CC increases the bias voltage Vb by the amount of drift, and the bias voltage is positioned at the optimum operating point Pop ′ of the drifted modulation curve LD. Control to correct to Vb ′ is performed.
[0005]
[Problems to be solved by the invention]
However, in the conventional LN control circuit CC described above, the current bias voltage Vb is directly obtained based on the DC voltage value detected by the monitor PD 3a. There was a problem that it was.
[0006]
In order to eliminate the above-described problems caused by the prior art, the present invention provides a light intensity modulation device capable of obtaining a drift value of the modulation curve L with high accuracy and controlling the bias voltage value with high accuracy with a simple configuration. The purpose is to provide.
[0007]
[Means for Solving the Problems]
In order to solve the above-described problems and achieve the object, a light intensity modulation apparatus according to claim 1 intensity-modulates an input optical signal by an input electric signal, and outputs the intensity-modulated light modulation signal to an external output. And a light intensity modulator having a monitor light detector for monitoring the light modulation signal, bias voltage applying means for outputting a bias voltage of the electric signal, and a pilot signal output for generating a pilot signal of a predetermined fundamental frequency Means for superimposing the bias voltage and the pilot signal and applying them to the light intensity modulator, and the predetermined fundamental frequency component and the predetermined fundamental frequency of the light modulation signal detected by the monitor photodetector. A filter that extracts the double harmonic component, and the ratio of the second harmonic component to the predetermined fundamental frequency component output from the filter are calculated. The results based on search of drift of the modulation curve of the light intensity modulator, characterized in that said bias voltage to offset the drift and a control means for generating said bias voltage applying means.
[0008]
According to the first aspect of the present invention, the superimposing means outputs the bias voltage of the electric signal output from the bias voltage applying means and the pilot signal of the predetermined fundamental frequency output from the pilot signal output means to the light intensity modulator, and the filter. Extracts the predetermined fundamental frequency component and the second harmonic component of the predetermined fundamental frequency from the optical modulation signal detected by the monitor photodetector, and the control means outputs the predetermined fundamental frequency output from the filter. The ratio of the second harmonic component to the component is calculated, the drift of the modulation curve of the light intensity modulator is obtained based on the calculation result, and the bias voltage that cancels the drift is generated in the bias voltage applying means I try to let them.
[0009]
According to a second aspect of the present invention, there is provided the light intensity modulation device according to the above invention, further comprising a sampling unit that samples the signal output from the filter in synchronization with the pilot signal, and the control unit outputs the sampling unit. The bias voltage is controlled based on the sampled value.
[0010]
According to a third aspect of the present invention, in the light intensity modulation device according to the above invention, the control means determines the direction of the drift based on the sign of the amplitude value of the second harmonic component, and the drift The bias voltage is controlled so that becomes zero.
[0011]
According to a fourth aspect of the present invention, there is provided the light intensity modulation device according to the above invention, wherein the control means includes a phase shift between a phase of the predetermined fundamental frequency component and a phase of the second harmonic component caused by a phase characteristic of the filter. And the bias voltage is controlled.
[0012]
According to a fifth aspect of the present invention, there is provided the optical intensity modulation device according to the above aspect, wherein the phase shift between the phase of the predetermined fundamental frequency component and the phase of the second harmonic component caused by the phase characteristic of the filter is corrected. A filter is provided in the subsequent stage of the filter.
[0013]
According to a sixth aspect of the present invention, there is provided the light intensity modulation device according to the above invention, wherein the control means and the phase of the predetermined fundamental frequency component until the control light is input from the monitor light detector to the control means. A plurality of phase regions for performing a calculation process corresponding to the phase shift from the phase of the double harmonic component is provided, and the drift direction is determined by the calculation process of the phase region corresponding to the position of the phase shift. .
[0014]
According to a seventh aspect of the present invention, there is provided the light intensity modulation device according to the above invention, wherein the control means includes the phase of the predetermined fundamental frequency component until the control light is input from the monitor light detector to the control means. A plurality of phase regions that perform calculation processing corresponding to the phase shift with the phase of the double harmonic component are provided in an overlapping manner, and calculation processing is performed for each phase region corresponding to the position of the phase shift, and each calculation processing result is obtained. And determining the direction of the drift.
[0015]
The light intensity modulation device according to claim 8 is characterized in that, in the above invention, the control means determines the direction of the drift by weighting each calculation processing result between overlapping phase regions. .
[0016]
According to a ninth aspect of the present invention, there is provided the light intensity modulation device according to the above invention, wherein the control means includes a light modulation signal detected by the monitor light detector generated by an arrangement position of the monitor light detector and the light intensity modulator. The relative drift with respect to the optical modulation signal output from the outside to the outside is controlled in addition to the drift.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
A light intensity modulation device according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram showing a schematic configuration of an optical transmission apparatus including an optical intensity modulation apparatus according to an embodiment of the present invention. In FIG. 1, this optical transmission device has a light intensity modulation device including an LN modulator 3 and an LN control circuit 5, and the LN modulator 3 is connected to an optical fiber 2 connected via a connection point P1. And connected to the optical fiber 4 via the connection point P3.
[0018]
The LN modulator 3 is the same as the LN modulator 3 shown in FIG. 12, and is a Mach-Zehnder type light intensity modulator using LiNbO 3. For example, an electrical signal S2 of 10 Gbps is input to the LN modulator 3 via the connection point P2, and the LN modulator 3 uses the electrical signal S2 to intensify the optical signal S1 transmitted from the LD1 connected to the optical fiber 2. The modulated optical signal S3 is output to the optical fiber 4.
[0019]
The LN modulator 3 has a monitor PD 3a that monitors a leaked light signal during intensity modulation, and is connected to the LN control circuit 5, and the signal S5 detected by the monitor PD 3a is connected to the LN control circuit via connection points P5 and P7. Output to 5 side. On the other hand, the signal S4 on which the controlled bias voltage Vb and the pilot signal Va for detecting the drift of the modulation curve L are superimposed is output to the LN modulator 3 side via the connection points P6 and P4.
[0020]
In the LN control circuit 5, the current-voltage conversion unit 11 converts the current value of the signal S5 input via the connection point P7 into a voltage value, and the bandpass filter 12 converts the pilot of the converted voltage signal. The signal Va component and the second harmonic component of the pilot signal Va are filtered and output to the amplifier 13. The amplifier 13 amplifies the input signal and outputs the amplified signal to the AD converter 14. The AD converter 14 converts the input analog signal into a digital signal and outputs the digital signal to the controller C.
[0021]
The controller C detects the drift of the modulation curve L based on the value of the digital signal sampled by the AD converter 14 and outputs a bias voltage control value that takes into account the drift value corresponding to this drift to the DA converter 15. . The analog value of the bias voltage control value output from the DA converter 15 is output to the adder 20 as the bias voltage Vb via the amplifier 16. On the other hand, a pilot signal Va which is a 1 kHz sine wave is output from a synthesizer 17 as an oscillator, amplified to a predetermined value via an amplifier 18 and then output to an adder 20. Note that, for example, a 1 kHz pilot signal output from the synthesizer 17 generates a synchronization signal of the pilot signal via the comparator 19 and is output to the controller C. The controller C performs the sampling control of the AD converter 14 and the DA converter 15 based on the synchronization signal input from the comparator 19. The adder 20 adds and superimposes the bias voltage Vb and the pilot signal Va described above, and outputs the resultant signal as a signal S4 to the LN modulator 3 side via the connection points P6 and P4.
[0022]
Here, the bias voltage control operation using the pilot signal Va by the controller C will be described. The drift value of the modulation curve L is “V0” (see FIG. 14), and the frequency of the pilot signal Va is “fp”. Therefore, the pilot signal Va is Vp · sin (2π · fp · t). The value of the control voltage “Vπ” is “1”. In this case, the signal S output from the bandpass filter 12 has Cf as a constant,
S = Cf · sin (2π · fp · t)
+ 0.25 · Cf · π ² · (Vb + V0) · Vp · cos (4π · fp · t) (1)
It can be expressed as The first term on the right side of Equation (1) is a sine wave of the frequency fp component (fundamental wave component), and the second term is a cosine wave of the second harmonic (2fp) component of the fundamental wave fp component.
[0023]
Here, the ratio α of the second harmonic 2fp component to the fundamental wave fp component is
α = 0.25 · π ² · (Vb + V0) · Vp (2)
It becomes. When the control voltage Vπ is not “1” in Expression (2), the ratio α is a value obtained by dividing the ratio α shown in Expression (2) by the value of the control voltage Vπ. As shown in FIG. 2, the dependence of the ratio α on (Vb + V0) exhibits a characteristic that (Vb + V0) decreases rapidly near 0, and the controller C obtains the value of (Vb + V0) by obtaining the ratio α. Can be sought.
[0024]
For example, as shown in FIG. 3, the AD converter 14 samples at eight sampling points SP0 to SP7 for one period of the fundamental wave fp, corresponding to the second harmonic 2fp, and at each sampling point SP0 to SP7. If the corresponding sampling values are sampling values x (0) to x (7), the fundamental wave fp component β1 is
β1 = 0.5√ ((x (0) −x (4)) ² + (x (2) −x (6)) ² ) (3)
And the second harmonic 2fp component β2 is
β2 = 0.25√ (((x (0) −x (2) + x (4) −x (6)) ²
+ (X (1) −x (3) + x (5) −x (7)) ² ) (4)
Can be obtained as Here, since the amplitude Vp of the pilot signal Va is known, the ratio α can be obtained using the equations (2) to (4), and the value of (Vp + V0) can be finally obtained.
[0025]
Here, since the bias voltage Vb = 0 when the LN control circuit 5 is turned on, the initial drift value V0 can be known from (Vb + V0) = V0 calculated by the controller C. Therefore, the controller C gives the DA converter 15 the initial bias voltage Vb = −V0 that should cancel out the drift value V0. The controller C performs control to apply a bias voltage Vb in which the value of (Vb + V0) is 0 in order to cancel the subsequent drift value V0. In this case, as shown in FIG. 2, since (Vb + V0) has a ratio α that suddenly decreases at 0, the bias voltage Vb for correcting the drift can be given with high accuracy.
[0026]
By the way, although the absolute value of the drift value V0 can be known when the power is turned on, since the equations (3) and (4) are calculation of the square root, the sign of the new drift value V0 is unknown. However, in FIG. 3, at the time of zero crossing of the fundamental wave fp, the sign of the value of the second harmonic 2fp having a positive drift value and the sign of the value of the second harmonic 2fp 'having a negative drift value. Are inverted, and the sign of the drift value V0 can be determined based on this relationship. That is, the positive / negative can be determined by the value γ using the sampling values x (0), x (2), x (4), and x (6).
γ = x (0) −x (2) + x (4) −x (6) (5)
When this value γ is positive, the drift value V0 is positive. Therefore, in order to cancel this drift value V0, the controller C outputs the bias voltage Vb to the DA converter 15 as a negative value, and when the value γ is negative. Since the drift value V0 is negative, the controller C outputs the bias voltage Vb to the DA converter 15 as a positive value in order to cancel out the drift value V0.
[0027]
In other words, the signal S corresponding to the expression (1) can be replaced by S = Asin (2πfpt) + Bcos (2π2fpt). In this embodiment, the sign of the amplitude value B of the second harmonic component is determined by the sign. The direction of drift deviation is determined.
[0028]
In this embodiment, the pilot signal Va is generated, the drift value V0 is determined based on the sampling value of the second harmonic 2fp generated by the nonlinearity of the modulation curve L, and the magnitude and positive / negative are determined, and the drift value V0 is corrected. Therefore, the operating point control with high accuracy can be performed.
[0029]
Incidentally, when the bandpass filter 12 is realized by an analog filter as shown in FIG. 4 in the above-described embodiment, phase rotation occurs when passing through the analog filter, and the fundamental waves fp and 2 depend on the frequency dependence of the phase characteristics of the analog filter. A phase shift from the double harmonic 2fp occurs (see FIG. 5B). FIG. 5A shows the relationship between the fundamental wave fp and the second harmonic 2fp when the analog filter is ideal and no phase shift occurs. Here, for example, when the fundamental wave fp is 1 kHz and the second harmonic 2fp is 2 kHz, at the sampling point A1 shown in FIG. 5, as shown in FIG. 6, the phase point m1 ( −162 °) and a phase shift m2 (147 °) of the second harmonic 2fp. Therefore, as shown in FIG. 7, it is preferable to provide a phase shifter 12 a that cancels this known phase shift in the subsequent stage of the band-pass filter 12. However, if the known phase shift value is a value that does not affect the above-described positive / negative determination of the drift value V0, it is not necessary to provide a phase shifter.
[0030]
Alternatively, the controller C may perform the calculation by adding a phase shift between the fundamental wave fp component and the second harmonic wave 2fp component without providing the phase shifter 12a described above. For example, the signal S described above can be expressed as S = Asin (2πfpt) + Bcos (2π2fpt). If the fundamental wave component has a phase φ2 and a second harmonic component phase φ2, the controller C = Asin (2πfpt + φ1) + Bcos (2π2fpt + φ2)
[0031]
As a first method for determining whether the drift value V0 is positive or negative, as shown in FIG. 8, the phase region is divided, and positive / negative determination processing corresponding to each division (regions E1 to E4) is performed. For example, when the phase shift is −π / 4 to π / 4 (region E1), the drift value V0 is determined to be positive when the value γ is positive, and the drift value V0 is negative when the value γ is negative. judge. When the phase shift is π / 4 to 3π / 4 (region E2), the value γ2 is
γ2 = −x (1) + x (3) −x (5) + x (7) (6)
When the value γ2 is positive, the drift value V0 is determined as positive, and when the value γ2 is negative, the drift value V0 is determined as negative.
[0032]
Further, when the phase shift is 3π / 4 to π and from −π to −3π / 4 (region E3), it is determined that the drift value V0 is negative when the value γ is positive, and the drift when the value γ is negative. The value V0 is determined to be positive. Further, when the phase shift is −π / 4 to −3π / 4 (region E4), the drift value V0 is determined to be negative when the value γ2 is positive, and the drift value V0 is positive when the value γ2 is negative. Is determined. As a result, in all phase shift regions, the controller C can make a positive / negative determination of the drift value V0 with the phase shift taken into account.
[0033]
As a second method for determining whether the drift value V0 is positive or negative, as shown in FIG. 9, the phase region is divided by allowing overlap, and two phase regions (regions E11 to E11 corresponding to the value of the phase shift). A positive / negative determination is made according to each determination result of E14). In addition, the positive / negative determination of area | regions E11-E14 is the same as each positive / negative determination of area | region E1-E4. For example, when the phase shift is 0 to π / 2, the positive / negative determination is made using the determination results of the regions E11 and E12. When the phase shift is π / 2 to π, the positive / negative determination is made using the determination results of the regions E12 and E13. Furthermore, when the phase shift is 0 to −π / 2, a positive / negative determination is made using the determination results of the regions E11 and E14. When the phase shift is −π / 2 to −π, positive / negative is determined using the determination results of the regions E13 and E14. Here, the positive / negative determination based on the determination result means that the controller C recognizes that the positive / negative determination result is correct only when, for example, two determination results match.
[0034]
As a third method for determining whether the drift value V0 is positive or negative, each determination result of the two phase regions is weighted in the second method. For example, when the phase shift is −π / 4 (point P10), that is, the phase shift of −π / 4 shown in FIG. 10B is generated from the state without the phase shift shown in FIG. 10A. In this case, each determination result of the region E11 and the region E14 is multiplied by cos (45 °) and sin (45 °), respectively, and weighted. Based on the result, positive / negative determination is performed.
[0035]
As another problem, the monitor PD 3a of the above-described embodiment is provided in the LN modulator 3 and detects light 41 leaking from the Mach-Zehnder type Y coupling portion PY as shown in FIG. However, the monitor PD 3a also detects the light 42 leaking from the light output waveguide 40 provided at the subsequent stage of the Y coupling portion PY. Here, the light leaks from the Y coupling portion PY when the optical signal S3 is off, and the monitor PD 3a detects light having a phase opposite to that of the optical signal S3. On the other hand, the light 42 leaking from the light output waveguide 40 with a small amount of light leaks when the optical signal S3 is on, and is in phase with the optical signal S3. For this reason, the light 42 interferes with the light 41, causing a phase shift in the signal detected by the monitor PD 3a. This phase shift changes due to variations in the arrangement position of the monitor PD 3a.
[0036]
FIG. 12 is a diagram showing a phase relationship between the optical output Po with respect to the bias voltage and the monitor current output Im detected by the monitor PD 3a. FIG. 12A shows the phase relationship between the optical output Po and the monitor current output Im when there is no phase shift. In this case, the controller C performs control such that (Vp + V0) = 0 so that the midpoint position Pc between the maximum value Immax and the minimum value Immin of the monitor current output is reached. On the other hand, FIG. 12B shows the phase relationship between the optical output Po and the monitor current output Im when a phase shift occurs. Even in this case, the controller C performs control such that (Vp + V0) = 0 so that the midpoint position Pc is reached, but the bias voltage at this time does not correspond to the midpoint position of the optical output Po, and the phase of ΔV Deviation has occurred, and a bias voltage that is an optimum operating point is not applied. In consideration of this phase shift, it is necessary to shift the bias voltage to the midpoint position Pc ′ of the optical output Po. In this case, the controller C may perform control such that (Vp + V0) = Cnt (a constant value) with the phase shift ΔV as a known drift value.
[0037]
【The invention's effect】
As described above, according to the present invention, the superimposing means outputs the bias voltage of the electrical signal output from the bias voltage applying means and the pilot signal of the predetermined fundamental frequency output from the pilot signal output means to the light intensity modulator. The filter extracts the predetermined fundamental frequency component of the optical modulation signal and the second harmonic component of the predetermined fundamental frequency from the optical modulation signal detected by the monitor photodetector, and the control means outputs from the filter A ratio of the second harmonic component to a predetermined fundamental frequency component is calculated, a drift of the modulation curve of the light intensity modulator is obtained based on the calculation result, and the bias voltage that cancels the drift is applied to the bias voltage. Since it is made to produce | generate by a means, there exists an effect that correction | amendment of drift can be performed with high precision with a simple structure.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an optical transmission apparatus including an optical intensity modulation apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram showing (Vb + V0) dependency of a ratio of a second harmonic to a fundamental wave which is a pilot signal.
FIG. 3 is a waveform diagram of a fundamental wave and a second harmonic.
FIG. 4 is a circuit diagram showing an example of a bandpass filter.
FIG. 5 is a waveform diagram showing a case where a phase shift occurs due to a bandpass filter.
FIG. 6 is a diagram illustrating frequency dependence of a phase by a bandpass filter.
FIG. 7 is a block diagram illustrating a schematic configuration of an optical transmission device including a phase shifter that cancels a phase shift of a bandpass filter.
FIG. 8 is a diagram illustrating an example of a region division for positive / negative determination corresponding to a phase shift.
FIG. 9 is a diagram illustrating an example of region division for positive / negative determination corresponding to a phase shift.
FIG. 10 is a waveform diagram showing an example of phase shift.
FIG. 11 is an explanatory diagram showing a state of light detection by the monitor PD.
FIG. 12 is a diagram showing a phase relationship between an optical output with respect to a bias voltage and a monitor current output detected by a monitor PD.
FIG. 13 is a block diagram showing a schematic configuration of a conventional optical transmission apparatus.
14 is an explanatory diagram illustrating bias control of the optical transmission device illustrated in FIG. 13; FIG.
[Explanation of symbols]
1 Laser diode (LD)
2,4 Optical fiber 3 LN modulator 3a Monitor PD
5 LN Control Circuit 11 Current Voltage Converter 12 Band Pass Filter 12a Phase Shifters 13, 16, 18 Amplifier 14 AD Converter 15 DA Converter 17 Synthesizer 19 Comparator 20 Adder 40 Light Output Waveguide 41, 42 Optical C Controller PY Y Coupling Connection points P1 to P7

Claims (8)

A light intensity modulator having a monitor light detector for intensity-modulating the input optical signal by the input electric signal, outputting the intensity-modulated light modulation signal to the outside, and monitoring the light modulation signal;
Bias voltage applying means for outputting a bias voltage of the electric signal;
Pilot signal output means for generating a pilot signal of a predetermined fundamental frequency;
Superimposing means for superimposing the bias voltage and the pilot signal and applying them to the light intensity modulator;
A filter for extracting the predetermined fundamental frequency component and the second harmonic component of the predetermined fundamental frequency of the optical modulation signal detected by the monitor photodetector;
The bias that calculates the ratio of the second harmonic component to the predetermined fundamental frequency component output from the filter, obtains the drift of the modulation curve of the light intensity modulator based on the calculation result, and cancels the drift Control means for causing the bias voltage applying means to generate a voltage;
The control means corrects a phase shift between the phase of the predetermined fundamental frequency component and the phase of the second harmonic component caused by the phase characteristic of the filter, and controls the bias voltage. Light intensity modulator.   A light intensity modulator having a monitor light detector for intensity-modulating the input optical signal by the input electric signal, outputting the intensity-modulated light modulation signal to the outside, and monitoring the light modulation signal;
  Bias voltage applying means for outputting a bias voltage of the electric signal;
  Pilot signal output means for generating a pilot signal of a predetermined fundamental frequency;
  Superimposing means for superimposing the bias voltage and the pilot signal and applying them to the light intensity modulator;
  A filter for extracting the predetermined fundamental frequency component and the second harmonic component of the predetermined fundamental frequency of the optical modulation signal detected by the monitor photodetector;
  The bias that calculates the ratio of the second harmonic component to the predetermined fundamental frequency component output from the filter, obtains the drift of the modulation curve of the light intensity modulator based on the calculation result, and cancels the drift Control means for causing the bias voltage applying means to generate a voltage;
  And a phase shifter for correcting a phase shift between the phase of the predetermined fundamental frequency component and the phase of the second harmonic component caused by the phase characteristic of the filter is provided in the subsequent stage of the filter. Intensity modulation device. Sampling means for sampling the signal output from the filter in synchronization with the pilot signal,
The control means, the light intensity modulator according to claim 1 or 2, characterized in that for controlling the bias voltage on the basis of the sampling values the sampling means is output. The control means determines the direction of the drift based on the sign of the amplitude value of the second harmonic component, and controls the bias voltage so that the drift becomes zero. Item 4. The light intensity modulation device according to any one of Items 1 to 3 . The control means divides a possible range of a phase shift value between the phase of the predetermined fundamental frequency component and the phase of the second harmonic component from the monitor light detector to the input to the control means. multiple is configured such that the value of the phase shift performs the arithmetic processing corresponding to the phase areas belonging among the phase regions, claim 1, characterized in that to determine the direction of the drift by the arithmetic processing ~ 5. The light intensity modulation device according to any one of 4 above. The control unit overlaps a range in which a phase shift value between the phase of the predetermined fundamental frequency component and the phase of the second harmonic component between the monitor light detector and the input to the control unit can take. Each of the plurality of phase regions divided so as to perform each arithmetic processing corresponding to each phase region to which the phase shift value belongs, and based on each arithmetic processing result, the direction of the drift is determined. light intensity modulation device according to any one of claims 1-4, characterized in that the determination. The light intensity modulation apparatus according to claim 6 , wherein the control unit weights each calculation processing result between overlapping phase regions and determines the direction of the drift. The control means adds a relative drift between an optical modulation signal detected by the monitor optical detector generated by an arrangement position of the monitor optical detector and an optical modulation signal output to the outside from the optical intensity modulator to the drift. controlling of the light-intensity modulation device according to any one of claims 1-7, characterized in.

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